This invention relates to the production of curved glass sheets. In particular the invention relates to the production of glass sheets having a curved shape of a vehicle window such as a windscreen or rear window. Such curved sheets may be toughened and used singly as a window, or either toughened or annealed and then used as a component of a laminated window such as a windscreen. Other uses for the curved glass sheets are for architectural purposes for example.
It is customary to cut a flat glass sheet for a vehicle window to a required shape, dictated by the styling of the vehicle in which the window is to be fitted, to smooth the cut edges of the sheet, and then suspend the sheet from a tong bar by means of tongs whose tong points grip the upper margin of the sheet at a series of gripping points which are spaced apart along the upper margin of the sheet. The flat glass sheets are heated and then bent and/or toughened while suspended from the tongs.
When the suspended sheet is bent, usually by closing horizontally moving bending dies on to the sheet, there are problems when the tongs move to follow the change of shape of the upper margin of the sheet. When the bending dies open the tongs which have been displaced are liable to exert forces on the sheet tending to distort its shape.
There is also a tendency for a suspended bent glass to change shape after the dies have opened, mainly due to temperature changes, with the result that after cooling by toughening or annealing, the shape of the sheet has changed from the shape imposed on the sheet by the bending dies. This is allowed for in the design of the shape of the die surfaces which close on to the sheet, in order to ensure that the bent sheet has a required shape when it has cooled to ambient temperature.
United Kingdom Patent Specification No. 473,604 describes a way of suspending a flat glass sheet from a series of tongs. The suspension for some of the tongs may be balanced by means of balance weights so that the tongs exercise an endwise or sideways pull on the flat glass sheet acting in the plane of the glass in order to keep the sheet flat.
An arrangement of tongs for suspending a glass sheet for bending is the subject of United Kingdom Patent Specification No. 1,185,355, in which the tongs are suspended in pairs from horizontal bars which can swivel in such a way that when the bending dies close on to the sheet the horizontal bars rotate about vertical axes to keep the suspension points for the tongs vertically above the top edge of the glass sheet, thereby avoiding horizontal force components exerted on the glass by the tongs during and after bending.
U.S. Pat. No. 3,888,635 describes a process in which bending dies are maintained at the same temperature as the glass temperature to which the glass is heated before bending, and the glass is supported by near-vertical rollers while it is heated and being transported to the bending dies. When the glass has been bent the upper edge of the glass is gripped by tongs so that the bent glass sheet is freely suspended when the dies open. This process produces curved glass sheets, both toughened and annealed, for inclusion in laminated assemblies.
There are problems associated with distortions introduced into the glass due to forces exerted on the glass sheet, particularly when the glass is released from the dies and becomes freely suspended from the series of tongs.
There are other glass bending and toughening plants in operation known as intermittent or multistage die bending plants, in which flat glass sheets are suspended on tongs throughout bending and toughening. The products from such plants are not used for laminating.
The current trend is to use thinner glass sheets in the production of motor-vehicle windows, and thinner glass is more vulnerable to distortion introduced by forces exerted on the glass by the tongs while the glass is still hot and deformable.
It is a main object of the present invention to provide a new method and apparatus for producing a curved glass sheet in which, either throughout the processing of the sheet, or at certain stages of the processing, the sheet is suspended from a series of tongs and the forces acting on the suspended glass sheet constitute a dynamic force system which acts on the sheet in such a way as to tend to change the shape of the sheet towards a desired shape, and/or applies forces to a bent glass sheet in such a way as to assist in maintaining that bent shape during further processing of the sheet, for example toughening or annealing.
It is another object of the invention to provide a method and an apparatus for producing toughened glass sheets and annealed glass sheets on a conventional intermittent die bending plant, the shapes of which sheets are sufficiently precise to enable them to be used on a commercial scale for the production of laminated glass windows.